Many methods exist for displaying on a viewing screen an image which has been picked up on a radiation-sensitive detector, e.g., an infrared (IR) detector. The screen is usually of the kind on which is produced an image housing adjacent lines or a raster similar to that used for displaying television pictures.
A video system is known, e.g., from U.S. Pat. No. 3,975,586, in which the output signal from a radiation-sensitive detector is converted to a video signal used for producing an image of the scene in front of the radiation detector. In order to present the loss of small details in the image, which are reproduced by high-frequency components in the video signal, in highlighted portions of the image, which are reproduced by low-frequency components in the video signal with high amplitude, it is proposed according to the above patent that the image contrast shall be improved such that the amplitude of the low-frequency components is limited, while the high-frequency components are left unchanged.
A drawback with the proposed amplitude limitation is that the difference in image intensity which would have been interesting to display cannot be displayed if the intensity falls outside the limitation. For example, if an image contains both sky and ground, and both have high intensity, no difference between these will be visible in the image, and thus the horizon can be invisible, which is detrimental in some cases.